1. Technical Field
The present disclosure relates to a radio-frequency (RF) communication apparatus and more, particularly, to a multi-band voltage-controlled oscillator (VCO) whose gain can be controlled depending upon a frequency band of interest, and an RF communication apparatus having the same.
2. Discussion of the Related Art
A local oscillator is included in a radio-frequency (RF) communication apparatus for conversion of signal frequency. A representative example of the local oscillator is a voltage-controlled oscillator (VCO) capable of changing an oscillation frequency according to a control voltage.
A large number of RF communication apparatuses operate in a broadband or a multi-band scenario. For example, a dual-band mobile telecommunication terminal has recently been introduced to support different frequency bands, such as a cellular band (around 800 Mhz band) and a PCS band (around 1800 MHz to 2 GHz). Thus, a VCO built into an RF communication apparatus that operates in a broadband or a multi-band, must also have a commensurate broadband operating frequency.
The VCO having a multi-band operating frequency generally includes a varactor whose capacitance varies according to the voltage applied thereto, in order to change the oscillation frequency of the oscillator. More specifically, the varactor is a device whose capacitance varies according to a bias voltage applied to a PN junction in the reverse direction. The range of tuning is very limited when using only the varactor, however, since the range of controlling the varactor is limited and active devices in the VCO have parasitic components. Accordingly, a VCO may be embodied to have a wide tuning range by including capacitors connected to switches and inductors connected to switches in the VCO. The inductors occupy a larger chip area than the capacitors, however, and continuous frequency tuning is difficult when the inductors are used.
Accordingly, a capacitance varying method is preferred to an inductance varying method.
A general VCO includes a coarse tune branch and a fine tune branch. A capacitance component in the coarse tune branch is adjusted by controlling a capacitor connected to a switch. The fine tune branch includes a varactor whose capacitance component varies according to a voltage and, thus, a capacitance component in the fine tune branch is changed according to the voltage. The VCO further includes an inductor in order to obtain a multi-band operating frequency by oscillation of an inductance component in the inductor and the conductance components in the coarse tune branch and the fine tune branch.
As the sizes of RF communication apparatuses have been reduced more and more, however, it is required to control a multi-band frequency signal with only a VCO, thus causing many problems.
In general, since a VCO that uses a fine tune branch has a fixed gain, it is difficult to control the gain of the VCO in a low frequency band and a high frequency band in different respective fashions. For example, when a VCO for a low frequency band generates a high-frequency signal, since the VCO for a low frequency band needs a large gain, the gain of the high-frequency signal is unnecessarily increased, thus increasing phase noise. The phase noise is a decisive factor that determines major performances of an RF circuit, such as selectivity and sensitivity. Accordingly, an increase in the phase noise causes a serious problem.